This invention relates in general to air conditioning and heating units for supplying conditioned air to the main duct work of large and small buildings and, more particularly, to a unit which contains high capacity filters for removing pollutants from the conditioned air.
Air conditioning and heating units are used to supply temperature conditioned air to the main duct work of buildings. One or more large fans within the unit are used to draw or blow supply air therethrough during which the heating or cooling coils in the unit provide temperature conditioning of the supply air. The supply air may comprise air which is recirculated from the building (i.e., return air), either alone or in combination with fresh air drawn from outside of the building. After the supply air has been heated or cooled, it is then discharged by the fan into the associated air distribution system. In other cases, the fan blows the air through the heating and cooling coils and then into the distribution system.
In addition to providing temperature conditioning of the air, units of this type must also maintain an acceptable level of air quality within the conditioned space. The levels of pollutants such as volatile organic compound and cigarette smoke which are generated within the building are typically controlled by a high capacity filtration system which removes airborne contaminants from the recirculated air. High capacity filtration devices such as mechanical filters and electronic and adsorptive devices are effective at removing undesired pollutants, but also typically cause a significant pressure drop across the filters. In order to compensate for the pressure loss, more fan energy must be utilized, thus significantly increasing the operating costs of the system. Replacement costs of high efficiency filters are higher than replacement costs of lower efficiency media, so action to extend the life of the high efficiency filter will remove system operating costs. The use of these extensive filtration systems has also become more prevalent as a result of increasingly stringent air quality requirements.
For example, in 1989 the Professional Society for Air Conditioning (ASHRAE) changed its ventilation standard by increasing the recommended amount of outside air to be incorporated into a ventilation system. ASHRAE increased the recommended amount by a factor of 4, from 5 cfm (cubic feet per minute) per person to 20 cfm per person for office buildings and from 5 cfm per person to 15 cfm per person in schools. Generally, a conditioning system consumes more energy when processing outside air, such as when changing its temperature, as compared to the energy consumed when processing return air, such as during filtration. Thus, increasing the recommended amount of outside air for use in a ventilation system, ASHRAE similarly increased the energy consumed by the ventilation system. For instance, an air conditioning system for an office building consumes between 10% to 15% more energy when processing 20 cfm per person of outside air, as compared to the energy consumed when processing 5 cfm per person.
The ASHRAE standard provides an alternative to increasing the amount of outside air. This alternative is referred to as the "indoor air quality procedure" (IAQ procedure) and is referred to as "demand control ventilation" (DCV). The IAQ procedure may usually substantially reduce the energy consumption of the conditioning system by allowing for variations in the percentage of outside and return air to be utilized. The IAQ procedure sets maximum contaminant levels acceptable within the occupied space. The DCV system is merely required to maintain contaminant levels within these acceptable maximums without mandating the use of 20 cfm per person of outside air.
The ASHRAE standard has been adopted by many states and national building codes. Presently, additional government agencies are becoming more involved in the regulation of indoor air quality within commercial buildings, and thus interest has increased in the IAQ procedure recommended by ASHRAE.
However, presently a conditioning system does not exist which adequately follows the IAQ procedure to maintain acceptable contaminant levels within a commercial building while minimizing the energy consumption of the conditioning system. In addition, previous systems which have attempted to comply with the IAQ standards have unnecessarily limited the life of the filter media since these foregoing systems draw excessive amounts of return and outside air through the high efficiency filters even when not required by the air quality. This excessive use shortens the high efficiency filter's life and increases the conditioning systems energy consumption. Such energy consumption results from the fact that high efficiency filters cause a substantial pressure drop within air passed therethrough. This pressure drop must be compensated for by the fans within the conditioning system, thereby consuming excess energy.
Moreover, often, fresh outdoor air which is drawn into the unit has a relatively high air quality and does not require the extensive filtering necessary with recirculated air. There are also occasions when the recirculated or return air is of sufficient quality to achieve the desired air quality standards without filtration. However, past systems have not been designed to bypass the filters when the incoming outside and/or return air has acceptable quality. Thus, the extensive filtration systems which are required to achieve necessary filtration at peak filtration loads are also used at non-peak times and thus cause significant unnecessary operating costs at non-peak times.
Finally, past systems have assumed outside air to be good quality and have used more outside air to dilute indoor contaminates without regard to outdoor air quality nor with regard to the cost of heating or cooling this additional outside air. The amount of outside air depends upon the quality thereof and upon the quality of the return air. Thus, maximizing the use of uncontaminated outside air could remove the need to filter the air and reduce filtering cost. However, increasing the amount of outside air may increase the amount of energy consumed to maintain a desired temperature. Thus, the filtering cost must be compared to the increased heating or cooling cost to select the most cost effective solution. Heretofore, no system has addressed these energy consumption concerns.
A need remains within the industry to provide an air conditioning system which overcomes the disadvantages noted above and experienced heretofore. It is an object of the present invention to provide such a system.